Add .../share/mk to the default system make path. This will fix the problem
with broken in-tree builds (which are used far more pervasively than I'd
known outside the tree). However, weird results may now happen if at any
point in the tree above you there happens to be a directory that has
subdirectory of share/mk, as unpredictable results will follow. This was
considered the lessor of the two evils, at least for now. In the future this
will be removed again when the underlying issues are resolved.

For MFCing, this is to ensure that when upgrading to head the share/mk
in-tree is always used.  Otherwise several build features will not work.
This passes a build using head's bmake which has this as the default
already.